From the early stage of providing voice-oriented services, a mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services. Various mobile communication standards such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution-Advanced (LTE-A), High Rate Packet Data (HRPD) of the 3rd Generation Partnership Project-2 (3GPP2), and IEEE 802.16 have recently been developed to support high-speed, high-quality wireless packet data communication services. In particular, the LTE system, which is a system developed to efficiently support high speed wireless packet data transmission, maximizes wireless system capacity by using various wireless access technologies. The LTE-A system, which is an advanced wireless system evolved from the LTE system, has enhanced data transmission capability as compared to the LTE system.
The existing 3rd generation wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, use technologies of an Adaptive Modulation and Coding (AMC) method and a channel-sensitive scheduling method to improve the transmission efficiency. With the use of the AMC method, a transmitter can adjust the amount of transmission data according to the channel state. That is, when the channel state is not good, the transmitter can reduce the amount of transmission data to adjust the reception error probability to a desired level. In contrast, when the channel state is good, the transmitter can increase the amount of transmission data to achieve efficient transmission of a large quantity of information while adjusting the reception error probability to a desired level.
With the use of the channel-sensitive scheduling-based resource management method, the transmitter selectively provides a service to a user having a good channel state among a plurality of users, thus increasing the system capacity compared to the method of assigning a channel to one user and providing a service to the user with the assigned channel. Such a capacity increase as in the above description is referred to as “multi-user diversity gain”. In short, the AMC method and the channel-sensitive scheduling method are methods that allow a transmitter to apply an appropriate modulation and coding technique at a point of time that is determined to be most efficient based on partial channel state information fed back from a receiver.
When the AMC method is used along with a Multiple Input Multiple Output (MIMO) method, the AMC method may include a function of determining a number of a spatial layer or a rank of a transmitted signal. In this case, in order to determine optimal data rate, the AMC method considers which number of layers is used in transmitting the signal by using the MIMO rather than simply considering a coding rate and a modulation manner.
In the wireless communication system, as one of technologies for improving an uplink performance, a multiple antenna method is applied. As a representative example, the LTE, which is a next generation mobile communication system of asynchronous cellular mobile communication standard group 3GPP, also improve a performance through a space diversity gain in an uplink by applying an antenna selective transmission diversity in the uplink based on a Single Carrier Frequency Division Multiple Access (SC-FDMA). In addition, the UE transmits the SRS so that the base station obtains information with respect to the uplink, the base station receives the SRS to obtain the channel state information with respect to the uplink bandwidth, and performs a frequency selective scheduling, a power control, a timing estimation and an MCS level selection base on the obtained channel state information. In addition, the base station may obtain downlink channel state information by using the SRS. In a Time Division Duplex (TDD) system, when a channel reversibility (i.e. a channel reciprocity) characteristic is used, the downlink channel may be estimated from the uplink channel information obtained through the SRS. Also, in a Frequency Division Duplex (FDD) system, long-term channel information of the downlink may be obtained through the SRS.
In the LTE system, the UE enables the base station to obtain the uplink channel information by transmitting the SRS. When the channel reversibility (i.e. the channel reciprocity) is used, the base station may estimate the downlink channel information from the SRS.
Meanwhile, an evolved communication system such as the LTE, may support a Central Antenna System (CAS) where an antenna in each of cells is disposed in a central area of a corresponding cell and a Distributed Antenna System (DAS) where a transmission/reception antenna is disposed in a central area of the cell and a plurality of distributed antennas are disposed in different areas in the cell, in each of the cells. In the existing system, one base station may transmit the downlink data and receive the uplink data, but in the CoMP system where the distributed antennas are disposed, the downlink data transmission and the uplink data transmission may be performed in different base stations or different antennas. In particular, when the SRS transmission is performed in different base stations or the different antennas, a macro base station may not receive the SRS transmitted by the UE. Thus, in the CoMP system using the distributed antenna, it is necessary to optimize the SRS transmission according to a destination of the SRS.